A heat transfer system, which is mounted to a vehicle for example, tends to have a cooling circuit. For example, the cooling circuit has a heat transfer medium, a heat exchanging part, and a heat transfer part. The heat transfer medium removes a heat at least from a heat source (e.g., an energy converter). The heat exchanging part dissipates the heat of the heat transfer medium to an outside of the heat transfer system. The heat transfer part delivers the heat transfer medium from the heat source to the heat exchanging part. With the above-described configuration, the heat is transferred from the heat source to the outside of the heat transfer system.